The present invention relates to the diagnostic imaging arts. The invention finds particular application in conjunction with volume CT imaging for medical purposes and will be described with particular reference thereto. However, it is to be appreciated that the present invention will also find application in conjunction with industrial, security, and other types of volume imaging apparatus and techniques.
In diagnostic imaging with CT scanners, a thin, fan shaped beam of radiation is projected from an x-ray source through a region of interest. The radiation source is rotated about the region of interest such that the same thin slice of the region of interest is irradiated from a multiplicity of directions spanning 360.degree.. In a third generation scanner, an arc of radiation detectors is mounted to the same gantry as the radiation source such that the two rotate together. In a fourth generation scanner, the x-ray detectors are mounted stationarily in a ring 360.degree. around the subject.
To image a volume of interest, a single slice image is typically generated. After a first slice image is generated, a subject support is indexed by a slice width generally on the order of a few millimeters, and another slice is generated. This slice image and index technique is repeated until slices spanning the volume of interest are generated. One drawback to this type of imaging is the relatively long time necessary to generate a large plurality of slices. Because the first and last slice are taken at a significantly different time, the volume image is distorted by a time evolution of the region of interest.
In spiral scanning techniques, the patient is generally moved continuously through the x-ray beam as the x-ray source rotates around the region of interest. In this manner, the fan shaped beam of radiation and the region of interest move in a spiral pattern relative to each other. The continuous motion is faster than indexing between slices, but still relatively slow.
In order to reduce the imaging time, some scanners collimate the beam of radiation into two slices. When the beam of radiation is collimated into two slices, two sets of radiation detectors disposed end to end are commonly provided. Typically, the thickness of the irradiated slice and the spacing between slices are adjustable. Such adjustments are relatively straight forward for two beams of radiation. However, the requirement that each beam of radiation strike only a single set of radiation detectors renders collimation into more than two beams mechanically awkward. Moreover, because the two beams originate from a common focal point, they are divergent, not parallel to each other. The divergent rays complicate and introduce errors into reconstruction techniques in which data is reconstructed into parallel slices. Moreover, as radiation from a single source is collimated into more beams, such beams become more widely divergent.
Systems have been proposed for examining the region of interest with a cone beam of radiation. However, cone beam image reconstruction is computationally intensive and slow. Moreover, cone beam imaging has a fixed resolution, based on detector size. Further, cone beam reconstructions tend to suffer from insufficiency of data problems, image artifacts, and other reconstruction errors.
The present invention contemplates a new, improved CT system which overcomes the above difficulties and others.